This invention relates to cryogenic containers for use at temperatures near to absolute zero, and, more particularly, to such containers made of a nonmetallic material to have low magnetic susceptibility.
In several branches of science, instruments or objects under study must be maintained at a low temperature, as within a few degrees of absolute zero. Superconducting instruments for measuring magnetic phenomena are operated at a temperature where superconductivity can occur, which for presently available commercial superconductor materials is typically less than about 10K., or about 450F. below zero. As might be expected, special techniques must be used to maintain instruments at such low temperatures for long periods of time.
In most laboratories, an instrument is maintained at such a low temperature by placing it into a bath of liquid helium, at a temperature of only 4.2K., or by placing it into an enclosure otherwise cooled to the superconducting temperature. The bath of liquid helium would vaporize almost instantaneously due to heat conducted from the surroundings, were the bath not maintained in a well-insulated container to reduce heat flow.
A commonly available container for holding such low temperature liquids is termed a Dewar flask, or simply a Dewar, after its inventor. A typical Dewar includes a metallic or glass inner container and a metallic or glass outer container, with a vacuum drawn on the space between the two containers. The surfaces of the containers are silvered, to reflect radiated heat and prevent radiative heat transfer, and the vacuum between the containers insulates against conductive and convective heat transfer. The Dewar is thus a sophisticated vacuum bottle for maintaining a cold interior.
Most commercially available Dewars are constructed either of metals or of thick, silvered glass. This construction is acceptable for many uses, but cannot be readily used in certain applications wherein instruments within the container are to measure small magnetic signals from sources outside the Dewar, and where the instrument should be placed as close as possible to the source of the signals. An instrument having these highly specialized requirements is a Neuromagnetometer.RTM. type biomagnetometer (manufactured by Biomagnetic Technologies, Inc., San Diego, CA), a highly sensitive magnetic measurement device capable of noninvasively measuring very small magnetic signals originating within the human body. The magnitude of the magnetic signals measured by this apparatus is typically less than 1/10,000,000 of the magnitude of the earth's magnetic field.
The instrumentation which measures the small magnetic field originating from the body must be operated at superconducting temperature, but also must be placed as closely as possible to the body of the patient being measured. The instrumentation must therefore be placed within a cryogenic container in the form of a Dewar or similar structure. However, the instrumentation cannot be separated from the body by a metallic or metal-coated structure as found in a conventional Dewar because of magnetic noise considerations. The container must neither generate magnetic artefacts, nor distort or attenuate the field being measured. It must have relatively thin walls to minimize the distance of separation of the instrumentation from the body. The container must be vacuum tight to hold an insulating vacuum therein, and must have sufficient mechanical stiffness and strength in its inner and outer walls to withstand normal handling forces and also the forces arising from the pressure differential across the walls.
There is therefore a need for a cylindrical cryogenic container which does not interfere with the taking of magnetic data from sources outside the container, is thin-walled, vacuum tight, and which may be used to maintain instruments at cryogenic temperatures. The present invention fulfills this need, and further provides related advantages.